This invention relates generally to the manufacture of esters, and more particularly to a process for the efficient, economic manufacture of such esters, and especially ethyl acetate, without the need of a conventional catalyst.
The production of many esters is accomplished by the reaction of an alcohol with a carboxylic acid. For example, ethyl acetate has long been produced commercially according to the following reaction: EQU C.sub.2 H.sub.5 OH+CH.sub.3 COOH.revreaction.CH.sub.3 COOC.sub.2 H.sub.5 +H.sub.2 O
This reaction is known to proceed slowly however, so that without the use of a catalyst to promote the forward reaction, equilibrium is established after several weeks at room temperature. Because this rate of reaction is not satisfactory for commercial applications a strong acid catalyst is often used to drive these reactions. For example, both concentrated sulfuric acid and hydrogen chloride gas have been used and are known to be particularly effective esterification catalysts in such reactions. These catalysts drive the reactions through to equilibrium more rapidly, i.e., within a matter of hours.
The use of such acids to catalyze esterification reactions may not be favored however. Acid catalysts also promote numerous side reactions to varying degrees resulting in small amounts of contaminants, some of which have very low sensory thresholds. As a result, synthetic ethyl acetate esters made from petroleum-based alcohols and carboxylic acids may have objectional sensory profiles.
In addition, acid catalysts are not acceptable for use in a particular and growing segment of the food industry which requires "naturally-made" esters for certain products. For example, "naturally-made" ethyl acetate is commercially useful for "natural decaffeination" processes for coffee. Other potential uses of natural esters include the preparation of certain pharmaceutical products and as direct food additives in natural flavor extracts.
Presently, such "naturally-made" esters are largely produced as by-products of certain fermentation processes. Unfortunately, the cost of manufacturing such "naturally-made" esters can be prohibitively high. For example, at current market prices, the cost of "naturally-made" ethyl acetate is approximately $10.00/lb. In addition, the purities of the product esters from such processes are unsatisfactory in many cases. For example, the purity of ethyl acetate commercially produced as a fermentation by-product rarely exceeds 90% to 92%. A variety of undesirable contaminants are also typically found in the resulting acetates when such processes are employed.
Accordingly, there is a need for a method of efficiently and economically producing esters without the aid of conventional acid catalysts. The present invention addresses that need.